1. Field of the Invention
The present invention relates to a method of removing impurities from wet process phosphoric acid. More particularly, the present invention relates to a process of precipitating aluminum, magnesium, iron and other cationic impurities from wet process phosphoric acid.
2. Description of the Prior Art
Wet process phosphoric acid is made by reacting phosphate rock with sulfuric acid. Phosphate rock is principally composed of fluoroapatite, but large amounts of contaminating substances are always present. These include silica and metal values such as iron, aluminum and magnesium along with smaller amounts of trace impurities. During the reaction with sulfuric acid, gypsum is precipitated and removed by filtration. After filtration of gypsum, many solubilized impurities remain in the acid. Because the quality of phosphate rock is declining as the better grades of the mineral are mined, the impurity levels in the wet process acid obtained from mined phosphate rock have been steadily increasing.
The presence of impurities in wet process phosphoric acid results in a variety of problems for those who manufacture ammonium phosphate fertilizers. One problem is that the presence of impurities dilutes the nitrogen and P.sub.2 O.sub.5 contents of the fertilizer ingredients thus resulting in a lower grade. Impurities also precipitate from the phosphoric acid and settle during storage thereby resulting in slime accumulation in storage tanks and other apparatus resulting in reprocessing and equipment cleaning costs, effective reduction in storage capacity, production loss, the clogging of liquid fertilizer application equipment, particularly spray nozzle orifices through which the fertilizer is applied, and precipitates which tie up phosphate in a form that is unavailable to plants (i.e., citrate insoluble form).
Because of the desirability of removing impurities from wet process acid, a number of different impurity removal methods have been developed. One such method is solvent extraction, of which there are a number of variations. Solvent extraction involves the extraction of either phosphoric acid or impurities from wet process phosphoric acid using an organic extractant, while leaving other components behind in the aqueous solution. Significant disadvantages of solvent extraction are the high capital and operating costs and the fact that organic solvents must be handled.
Another type of impurity removal process is concentration/clarification of wet process phosphoric acid. In the first step of this process, wet process acid is concentrated to about 54% P.sub.2 O.sub.5 and some of the impurities are allowed to precipitate. However, there are several major disadvantages to the process, namely, high energy consumption, difficulties in concentrating the acid from low quality phosphate rock, the cost of clarification equipment and the inability to reduce impurity concentrations to acceptably low levels.
Another general method of removing metal impurities from wet process acid involves ammoniation of wet process acid. Metal impurities normally found in phosphate rock include minerals containing magnesium, aluminum, iron and others.
U.S. Pat. No. 4,136,199 discloses a method for removing metal ion impurities from wet process phosphoric acid by treating waste pond water from a wet process phosphoric acid plant with lime or limestone to obtain a sludge high in calcium fluoride which, when added to wet process acid, causes the precipitation of solids containing fluorine and metal ions such as magnesium and aluminum.
U.S. Pat. No. 4,500,502 discloses a method of preparing a purified ammoniated phosphoric acid composition by reacting wet process phosphoric acid containing magnesium, aluminum and fluorine impurities with ammonium ions, including ammonia, in order to precipitate complex metal salts containing magnesium, aluminum and fluorine. Unfortunately the complex metal salts also contain valuable phosphates which are lost.
U.S. Pat. No. 4,299,804 discloses a method of removing magnesium and aluminum impurities from wet process phosphoric acid. The disclosed process maintains the Al/Mg mole ratio within the range of 1.1-2.0 and the F/Al ratio within the range of 3.5-7 by adding aluminum ion donating compounds (such as alum) and fluoride ion donating compounds (such as hydrofluoric acid) which are reacted with wet process acid to precipitate crystalline compounds containing magnesium, aluminum and fluorine (MgAl.sub.2 F.sub.8).
U.S. Pat. No. 4,493,820 discloses a typical concentration/clarification process.
U.S. Pat. No. 4,435,372 discloses the removal of aluminum, magnesium and fluoride ion impurities from wet process phosphoric acid by hydrolizing and recycling the off-gas scrubber solutions in the presence of a ferric iron catalyst.
U.S. Pat. No. 4,325,327 discloses a two-stage ammoniation process involving the precipitation of solids in the pH regions of 1.5-2.5 and 4-5. The solids are filterable and thus easily separated from the monoammonium phosphate solutions.